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BR24A Datasheet(PDF) 14 Page - Rohm

Part No. BR24A
Description  High Reliability Series EEPROMs I2C BUS
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Maker  ROHM [Rohm]
Homepage  http://www.rohm.com
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BR24A Datasheet(HTML) 14 Page - Rohm

 
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BR24A□□-WM series
Technical Note
14/17
www.rohm.com
2009.08 - Rev.C
© 2009 ROHM Co., Ltd. All rights reserved.
Cautions on microcontroller connection
Rs
In I
2C BUS, it is recommended that SDA port is of open drain input/output. However, when to use CMOS input / output of
tri state to SDA port, insert a series resistance Rs between the pull up resistance Rpu and the SDA terminal of EEPROM.
This is controls over current that occurs when PMOS of the microcontroller and NMOS of EEPROM are turned ON
simultaneously. Rs also plays the role of protection of SDA terminal against surge. Therefore, even when SDA port is
open drain input/output, Rs can be used.
Maximum value of Rs
The maximum value of Rs is determined by the following relations.
(1)SDA rise time to be determined by the capacity (CBUS) of bus line of Rpu and SDA should be tR or below.
And AC timing should be satisfied even when SDA rise time is late.
(2)The bus electric potential
A to be determined by Rpu and Rs the moment when EEPROM outputs 'L' to SDA bus
should sufficiently secure the input 'L' level (VIL) of microcontroller including recommended noise margin 0.1VCC.
Fig.54 I/O circuit diagram
Minimum value of Rs
The minimum value of Rs is determined by over current at bus collision. When over current flows, noises in power source
line, and instantaneous power failure of power source may occur. When allowable over current is defined as I, the following
relation must be satisfied. Determine the allowable current in consideration of impedance of power source line in set and so
forth. Set the over current to EEPROM 10mA or below.
Fig.55 I/O circuit diagram
Microcontroller
EEPROM
'L' output
RS
RPU
'H' output
Over currentⅠ
VCC
RS
VCC
I
10×10-3
I
RS
300[Ω]
Example)When VCC=3V, I=10mA
RS
3
Example)When VCC=3V, VIL=0.3VCC, VOL=0.4V, RPU=20kΩ,
×
×
20×103
1.67[kΩ]
RPU+RS
(VCC-VOL)×RS
+VOL+0.1VCC≦VIL
RS
from(2),
RPU
VIL-VOL-0.1VCC
1.1VCC-VIL
1.1×3-0.3×3
0.3×3-0.4-0.1×3
RS
RPU
Microcontroller
RS
Fig.52 I/O circuit diagram
Fig.53 Input / output collision timing
EEPROM
'L' output of EEPROM
'H' output of microcontroller
Over current flows to SDA line by 'H'
output of microcontroller and 'L'
output of EEPROM.
SCL
SDA
RPU
Microcontroller
RS
EEPROM
IOL
A
Bus line
capacity CBUS
VOL
VCC
VIL


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